A power IC such as a linear/switching regulator includes a sensing block which is connected to a feedback loop for control of circuit operation and sensing an output voltage. The sensing block determines whether or not the sensed output voltage corresponds to a desired voltage value, and then adjusts the output voltage through a logic circuit.
FIG. 1 is a circuit diagram of a related power IC including such a sensing block. As shown in FIG. 1, the related power IC connects an M:1 matched power transistor Tp and a sense transistor Ts to a common control signal line. In addition, the power transistor Tp is connected between a switch and a power ground, and the sense transistor Tp is also connected between the switch and the power ground via a sense resistor Rs. Accordingly, when current is applied to the power IC via the switch, sensing current corresponding to 1/M of power current Io flowing through the voltage transistor Tp flows through the sense resistor Rs according to a transistor matching ratio (M), and a voltage across the sense resistor Rs is applied as a sense voltage to a comparator Comp.
In addition, the comparator Comp is applied, as a reference voltage VREF, with a voltage from a reference voltage source VBG divided by a first resistor R1 and a second resistor R2 connected between the reference voltage source VBG and an analog ground. The power IC may be designed to separate the power ground from the analog ground to prevent power elements connected to the power ground from affecting analog elements connected to the analog ground.
Accordingly, the comparator Comp compares the sense voltage with the reference voltage and applies a result of the comparison to a logic circuit connected to an output port, and the logic circuit changes a final output of the power IC using the applied result.
The comparator can sense and change an output at a point of time when a voltage RsIo/M across the sense resistor Rs becomes equal to the reference voltage VREF. Accordingly, the maximum of current Io which can be sensed by the comparator Comp is determined by the following equation.
                              I          O                =                              M                          R              S                                ⁢                      V            REF                                              [                  Equation          ⁢                                          ⁢          1                ]            
As shown in FIG. 2, however, an unintended parasitic resistive component Rp exists in an actual power ground. Such a parasitic resistance Rp is generated by various factors such as a ground pattern, a pad, a bonding, a metal pattern and the like. Accordingly, it is difficult to predict a value of the parasitic resistance Rp which is generated during a process.
FIG. 2 shows an effect of such parasitic resistance Rp on the power IC. Power current Io passing through the power transistor Tp and sense current Io/M passing through the sense transistor Ts pass through the parasitic resistance Rp. Accordingly, since a voltage is applied across the sense resistance Rs, a sense voltage applied to the comparator Comp is determined by the following equation.
                    Vs        =                                            Io              M                        ⁢            Rs                    +                                                    (                                  M                  +                  1                                )                            M                        ⁢            IoRp                                              [                  Equation          ⁢                                          ⁢          2                ]            This equation can be rearranged as the following equation.
                    Io        =                              M                          Rs              +                                                (                                      M                    +                    1                                    )                                ⁢                Rp                                              ⁢          Vs                                    [                  Equation          ⁢                                          ⁢          3                ]            
In comparison with Equation 1, a term ‘(M+1)Rp’ is added to the denominator of Equation 3, and accordingly sensible current Io becomes small due to the parasitic resistance Rp in an actual circuit. Since a matching ratio (M) is typically more than 100, this has an effect on the sense current Io even if the parasitic resistance Rp has a small value. As a result, a difference between actual sense current and sense current calculated in design is generated due to the parasitic resistance Rp, which may result in low reliability of circuit operation.